Nomadic Planets May Swarm the Galaxy

May 5, 2015

Much public attention has focused on the recent discoveries of many hundreds of planets around other stars. A group of KIPAC scientists has now estimated that there may be up to ten thousand times as many planet-sized objects flying freely through our Galaxy as there are planets orbiting stars. They explore the implications for future sky surveys such as LSST, as well as our view of planet formation and even the origin of life.

Artist's conception of a Jupiter-sized nomad. Image courtesy of Wikimedia.

One of the many exciting developments in astronomy in recent years has been the rapid accumulation of knowledge about planets outside of our solar system, known as extrasolar planets or 'exoplanets'. As recently as 22 years ago there were zero planets known to mankind outside of our solar system, and it was a topic of debate whether any in fact existed. Today, thanks to ground-based observations and NASA's Kepler satellite, which look for miniscule light changes or wobbles of stars, some 2000 probable exoplanets are known to exist around other stars, with many new candidates discovered every month.
Into this already rapidly developing field, several KIPAC scientist have jumped in with their own planetary news, that in addition to these other solar systems, there are likely many times more planet-sized objects which are freely floating throughout our Galaxy, unbound to any star. The team, led by KIPAC postdoc Louie Strigari, and featuring postdoc Matteo Barnabe, KIPAC director Roger Blandford, and former KIPAC postdoc Phil Marshall, currently with Oxford University in the UK, calculated bounds on the number of such 'nomad' objects with several techniques.
A major input was from recent surveys of gravitational microlensing, where the light we receive from a star is briefly enhanced by an object passing between the star and the observer, due to the light bending effects of general relativity, a phenomenon first anticipated by Einstein in 1912.  These observations have recently revealed the presence of a number of unbound nomads. Because microlensing observations are likely to detect only the largest planetary bodies, there is a large amount of uncertainty on how many of the presumably much more numerous small unbound nomads there may be. Strigari and colleagues have used microlensing and other observations to estimate that the ratio of nomads to main sequence stars is at least 5, but could be much higher, approaching 10,000, depending on how small such planets can be.
Given the abundance of unattached nomads flying around the Galaxy, the KIPAC team set out to predict the effect on future astronomical surveys, which will observe many millions of stars and possibly see many of their microlensing effects on stars. These estimates included models of the spatial distributions of stars and nomads throughout the Galaxy, and the brightnesses of stars and masses of the nomads. They then folded in the sky coverage and sensitivity of upcoming surveys.
The authors propose that upcoming surveys will see many nomad microlensing events and will be able to determine the number and distribution of nomads with greatly improved precision. WFIRST, NASA's future infrared space telescope, given targeted observations toward the presumably nomad-dense inner part of the Galaxy, will be able to determine the ratio of nomads bigger than Jupiter to main sequence stars to a precision of 13%. Complimenting WFIRST's observations, systematic large area surveys such as LSST, in which KIPAC is a major participant, will detect microlensing events from nomads in outer parts of the Galaxy as well. Together, WFIRST, LSST, and other surveys such as the GAIA satellite will greatly constrain the total number and size distribution of nomads.
It seems likely that the largest nomads were formed in the same manner as stars, collapsing from a disk of proto-stellar matter. However, it seems that most would have to have formed in solar systems and then been ejected, flung into space by interactions with other bodies. Thus, constraining the distribution and nature of nomads would provide crucial input to our understanding of planet formation and solar system evolution. If nomads extend to small Pluto-sized masses and their number is at the large end of what the KIPAC team has estimated, then collisions between nomads or pieces of nomads and bound planets could be common enough to suggest that material and even microscopic life or its building blocks could be exchanged between distant solar systems.
This work is based in part on a paper published submitted to The Astrophysical Journal and available from arXiv at 1201.2687.
Science Contact:
Louie Strigari